This invention relates to the recovery of antimony in the form of antimony pentachloride from a spent antimony halides catalyst mixture, and more particularly to a process for the recovery of antimony pentachloride, which may readily be reused as a catalyst for the fluorination of chlorinated hydrocarbons, from an antimony catalyst mixture used for the fluorination of chlorinated hydrocarbons.
Antimony pentachloride serves as a catalyst for the fluorination, with hydrogen fluoride, of chlorinated hydrocarbons including, for example, carbon tetrachloride, tetrachloroethylene, chloroform, trichloroethylene and the like. Fluorinated chlorohydrocarbons are widely used as solvents, refrigerants and propellants in the aerosols industries. Such an antimony catalyst is generally in the form of liquid and contains partly-fluorinated antimony and trivalent antimony. In the fluorination process, the liquid antimony catalyst is charged into a reaction vessel wherein it acts on chlorinated hydrocarbons and hydrogen fluoride introduced therein to promote the fluorination of the chlorinated hydrocarbons. During the course of employment over a long period of time, however, various impurities or other substances are accumulated in the liquid catalyst, including small amounts of impurities introduced with the chlorinated hydrocarbons such as chlorohydrocarbons, water and the like; small amounts of high boiling point or high sublimating point chloro- or chloro-fluoro-hydrocarbons secondarily produced during the contact reaction; and inorganic metal salts such as of nickel, chromium iron, and the like, the metals being mainly derived from the materials of the reaction vessel. This lowers the catalytic activity and renewal of catalyst becomes necessary when its activity has become too low. The spent antimony catalyst mixture mainly composed of antimony compounds is generally passed to waste disposal at some cost. As a matter of fact, recovery of useful substances from the spent antimony catalyst mixture is desirable and advantageous from a standpoint of prevention of environmental pollution and protection of natural resources and as well as from an economic viewpoint.
The spent antimony catalyst mixture comprises pentavalent antimony halides as its primal components but antimony is also present in the form of trivalent compounds. (It is said that the pentavalent antimony halides are present either in the form of a mixture of SbCl.sub.5 and SbF.sub.5 or in the form of a mixture of partly-fluorinated antimony chlorides as expressed by the formula SbCl.sub.x F.sub.y where x + y = 5) Of these, certain antimony halides having high melting points such as SbCl.sub.3 (m.p. 73.degree. C), SbCl.sub.4 F (m.p. 75.degree. C) and SbCl.sub.3.7 F.sub.1.3 (m.p. 79.degree. C) have a tendency to block up distillation apparatus. In addition, the spent catalyst mixture also contains salts of nickel, chromium, iron, etc., sublimate substances with high sublimation points or high melting point substances such as hexachloroethane (sublimation point 185.degree. C), pentachlorofluoroethane (m.p. 100.degree. C) and the like (which also tend to block up distillation apparatus), or small amounts of the starting materials for the fluorination reaction and the reaction products. Accordingly, it is very difficult to recover useful substances, particularly antimony pentachloride, from the used antimony catalyst for reuse as catalyst. Antimony pentachloride can be usually readily isolated by distillation if not contained in a complicated mixture. When antimony pentachloride is present in a composition containing a variety of substances such as indicated above, the isolation of antimony pentachloride by mere distillation is extremely difficult due to the adverse effect of substances which block up the distillation apparatus.
Several processes for the recovery of antimony pentachloride from spent antimony catalyst mixtures have been heretofore proposed. For example, in U.S. Pat. No. 3,760,059, there is disclosed a process wherein a spent antimony catalyst mixture is mixed with trichloroethylene and heated to reduce all of the antimony compounds contained in the catalyst mixture into antimony trichloride, and the antimony trichloride is then crystallized in organic solution and separated, followed by coversion to antimony pentachloride by oxidation. However, this process undesirably involves offsetting steps to be diserably omitted if possible, such as the reduction of antimony compounds into the trivalent antimony compound and the oxidation of the antimony trichloride into antimony pentachloride, and also involves the steps of the crystallization and separation of antimony trichloride which are complicated and result in a relatively great loss.
The process of U.S. Pat. No. 3,784,671 comprises the steps of: (a) heating a spent antimony catalyst mixture to convert antimony fluorides contained therein to corresponding antimony chlorides and distilling off halogenated hydrocarbons for concentration; (b) thermally decomposing antimony pentachloride into antimony trichloride and chlorine; (c) separating organic impurities in the mixture by extraction with a halogenated hydrocarbon; (d) subjecting the antimony trichloride-containing residue to reaction with chlorine to form antimony pentachloride; and (e) recovering the thus formed antimony pentachloride by vacuum distillation. Though the above process represent an improvement in that the chlorine generated upon the thermal decomposition is recycled in the subsequent chlorination step without suffering a loss of the chlorine, it also involves offsetting steps of removing chlorine from antimony pentachloride to yield antimony trichloride and of subjecting the trichloride to oxidation reaction with chlorine to give antimony pentachloride. In addition thereto, the process requires such a complicated solid-liquid separation step wherein organic impurities are separated from antimony trichloride by dissolving them in a halogenated hydrocarbon.